Recently, demand has increased for providing mobile communications and easily deployed network communications. Much attention has been focused on RF and IF wireless communications systems and techniques for providing such types of communications. However, the RF communication spectrum is rather crowded. Therefore, many developers have recently focused on providing communications using the IR spectrum.
The IR communications channel proposes a number of challenges for the developer. Most notably, the environment in which a typical IR wireless communications system is to be deployed, such as an office building, warehouse or other enclosed structure, is fraught with interference sources such as the sun and lamps (fluorescent, filament, inverter fluorescent, etc.). In addition, it is desirable for some IR wireless communications systems to reduce the amount of power dissipated in the wireless communicating terminals. For example, in the context of portable transceivers, it is desirable to reduce the amount of power dissipated in transmitting information to conserve battery power.
The prior art has proposed pulse-position modulated transceivers for both wireless and wired communications. See H. K. Lu, T. H. Taur, K. C. Chen, C. K. Wang & M. T. Shih, Prototyping an Indoor High Speed Diffuse Infrared Transceiver for Wireless Data Communications, SINGAPORE ICCS/'94, p. 338-342 (1994); M. Audeh, J. Kahn & J. Barry, Performance of Pulse-Position Modulation on Measured Non-Directed Indoor Intrared Channels, IEEE Trans. On Comm., vol. 44, no. 6, June, 1996, p. 654-659; M. Rittler, F. Gfeller, W. Hirt, D. Rogers & S. Gowda, Circuit and System Challenges in IR Wireless Communication, ISSCC96, Feb. 10, 1996, SP 25.1; K. Yamazaki, On a New Detection Scheme of Optical PPM Signal, PROC. INT'L SYMP. INFOR. THEORY (1995); and U.S. Pat. No. 4,584,720. In such systems, the data to be communicated is formulated into a non-return to zero (NRZ) signal. Such an NRZ signal is pulse position modulated onto a carrier signal to produce a pulse position modulated (PPM) signal. The PPM signal includes variably spaced pulses, the relative location of which depends on transitions of the NRZ signal from a low level to a high level or from a high level to a low level. In particular, the PPM signal is divided into fixed length groups having 2.sup.i pulse positions or "slots" per group. Depending on the corresponding signal level of the NRZ signal, a pulse is selectively inserted into a particular slot of the group. That is, a train of pulses is produced in selected slots of the PPM signal, whose phase depends upon the polarity of the NRZ signal at the corresponding time.
The PPM signal thus formed may be inputted to a light emitting diode and transmitted in electromagnetic form to a receiver. The receiver has a photo diode which receives the electromagnetic PPM signal and converts it to electrical (i.e., voltage/current) form. The receiver amplifies the PPM signal, and may use an automatic gain control (AGC) circuit to vary the gain of the amplifier. A demodulator demodulates the NRZ signal from the PPM signal. The demodulator may employ a phase locked loop and/or slot clock to synchronize with the received PPM signal. The recovered NRZ signal may then be further processed, e.g., error corrected, processed, etc.
It is desirable to provide a simple and expandable PPM transceiver with a simple PPM encoder and a robust receiver that quickly locks onto, i.e., synchronizes with, a received PPM signal.